Question

average atomic mass
why?
most elements have more than one naturally - occurring isotope. as you learned previously, the protons of
atoms of an element have the same atomic number (number of protons), making them belong to the same ele -
ment, but they have different mass numbers (total number of protons and neutrons) giving them differ -
ent atomic masses. so which mass is put on the periodic table? this activity will help answer that question.
model 1 - a strip of magnesium metal

1. write in the atomic number for each mg atom in model 1.

12

1. what are the mass numbers of the naturally - occurring isotopes of magnesium shown in model 1?

24, 25, 26

1. do all of the atoms of magnesium in model 1 have the same atomic mass? explain.

no isotopes

1. for the sample of 20 atoms of magnesium shown in model 1, draw a table indicating the mass

numbers of the three isotopes and the number of atoms of each isotope present.

1. which isotope of magnesium is the most common in model 1?
2. based on model 1 and the table you created in question 4, for every 10 atoms of magnesium,

approximately how many atoms of each isotope will be found?

Explanation:

Step1: Recall atomic number concept

The atomic number of an element is unique and represents the number of protons. Magnesium's atomic number is 12.

Step2: Identify magnesium isotopes

Common naturally - occurring magnesium isotopes have mass numbers 24, 25, 26.

Step3: Understand isotope concept

Isotopes of an element have the same atomic number but different mass numbers due to different numbers of neutrons. So atoms of magnesium in Model 1 (isotopes) do not have the same atomic mass.

Step4: Create a table (assuming counts from visual inspection of Model 1)

Mass Number	Number of Atoms
25	6
26	2

Step5: Determine the most common isotope

From the table in Step 4, the isotope with mass number 24 has the highest count, so it is the most common.

Step6: Calculate for 10 atoms

Based on the ratio from the table in Step 4, for 10 atoms:

• For mass - number 24: $\frac{12}{20}\times10 = 6$ atoms
• For mass - number 25: $\frac{6}{20}\times10 = 3$ atoms
• For mass - number 26: $\frac{2}{20}\times10 = 1$ atom

Answer:

1. 12
2. 24, 25, 26
3. No, because they are isotopes.

4.

Mass Number	Number of Atoms
25	6
26	2

1. The isotope with mass number 24.
2. Mass number 24: 6 atoms; Mass number 25: 3 atoms; Mass number 26: 1 atom